Explanation:

Negative Slope:
A negative slope means that the curve is sloping downwards from left to right. This indicates that as we move along the curve from left to right, one variable decreases while the other variable increases.

Cannot Interest Each Other:
Two curves cannot intersect each other when they represent the relationship between the same two variables.

Indifference Curves:
Indifference curves are a graphical representation of a consumer's preference for different combinations of two goods. They show all the combinations of two goods that give the same level of utility or satisfaction to the consumer. Indifference curves have a negative slope because of the law of diminishing marginal rate of substitution.

Why Indifference Curves have a negative slope and cannot intersect:
- Indifference curves have a negative slope because of the law of diminishing marginal rate of substitution. This law states that as a consumer consumes more of one good, the marginal utility of that good decreases while the marginal utility of the other good increases. Thus, to maintain the same level of satisfaction, the consumer needs to consume more of the other good. This results in a negative slope of the indifference curve.
- Indifference curves cannot intersect because each curve represents a unique level of satisfaction. If two curves intersect, it would mean that the consumer is indifferent between two different levels of satisfaction, which is not possible.

Conclusion:
Therefore, the correct option is C, as indifference curves have a negative slope and cannot intersect each other.